I have my package defined like this:
<Package Compressed="yes" InstallerVersion="301" Languages="!(loc.LANG)" SummaryCodepage="!(loc.CODEPAGE)" Platform="$(var.MyPlatform)" />
Everything compiles fine, but I get a warning from Visual Studio about the invalid XML schema
The 'Platform' attribute is invalid - The value '$(var.MyPlatform)' is invalid according to its datatype 'NmToken' - The '$' character, hexadecimal value 0x24, cannot be included in a name.
This is just IntelliSense in Visual Studio complaining. We can't control that. It doesn't affect compiling or linking.